We pressed hard, with France, for an end to the EU arms embargo so that weapons might be sent to the rebels. Then, when those same rebels promptly pulled out of the peace talks due to take place later this month – leaving a question mark over the first hope of diplomatic progress for 12 bloody months – the Foreign Secretary pulled back, stressing that a decision would be made only after the conference.

The Syrian National Council’s vacillations cannot all be laid at our door. The largest rebel group blamed its withdrawal on Damascus’s growing reliance on Lebanon-based militants. In-fighting within the SNC itself is another factor. But to raise the prospect of military assistance is hardly to encourage the rebels to the negotiating table. Yet, for all the brutality of Bashar al-Assad’s regime, a negotiated solution must be preferable to what would be a protracted civil war even with international support.

Thus, a tactic to press Mr Assad to Geneva made it less likely that his opponents will attend. Meanwhile, Russia has said that its delivery of S-300 anti-aircraft missiles to Damascus will go ahead – to discourage outside interference, according to the deputy Foreign Minister. If the arms embargo was to be used as a bargaining chip, the plan badly backfired. If it was not, the implications were not thought through.

Behind the public confusion lies private disarray. Although much of parliament is hostile to British intervention, the Government is divided. David Cameron– his head turned by his hero’s welcome in Libya after the fall of Gaddafi – is the strongest proponent, backed by Michael Gove and, to a lesser extent, William Hague. Ranged against them are “doves” such as Nick Clegg, Justine Greening and Chris Grayling. It is they who are right, and the Prime Minister who is wrong.

As the situation worsens, the case against intervention is stronger than ever. Not only are yet more guns no solution, particularly with the rebels so fragmented and a growing proportion of them linked to al-Qa’ida. The increasingly sectarian conflict is also threatening to destabilise the entire region, as Sunni rebels bankrolled by Qatar and Saudi Arabia battle a Shia regime backed by Hezbollah and Iran.

The re-taking of Qusair by government troops yesterday is a case in point. The victory was achieved only with the help of Hezbollah, and it was met with both congratulations from Iran and warnings from a chief rebel commander that his forces will take the fight into Lebanon if necessary. Western involvement, even at arm’s length, will only add to an already impossible mix of politics, history and religion.

Might the use of chemical weapons change the calculation? Perhaps. But although Barack Obama has been charged with irresolution for not yet acting on his “red line” warning, he is right to be cautious. Evidence of the use of nerve agents is mounting – not least with yesterday’s confirmation that bodily fluid samples smuggled out of Syria contained sarin. The source of the attacks remains unknown, however. And while the most likely culprit may be the Assad regime, the consequences would be so momentous – targeted airstrikes on known missile stores, say – that assumption is not enough.

The scale of the suffering in Syria is appalling and the reports of atrocities, on both sides, ever more sickening. Nor is the end in sight. While the rebels’ loss of Qasair is a blow, it will not break the stalemate; and Russian intransigence leaves the only defensible international on-the-ground intervention – via UN blue helmets – out of reach. The sole hope of progress is, therefore, diplomatic. It is on the Geneva conference that the British government should be focusing its efforts. Arming the rebels is a simplistic fantasy, and it is distracting attention from the only sensible course.